1. Field of the Invention
The invention relates to a device for write precompensation of data signals to be recorded on a magnetic medium, the device having an input for receiving input data signals and an output for supplying the output signals to be recorded, at least a plurality of the signal transitions in these output data signals being delayed with respect to the corresponding signal transitions in the input data signal.
When certain data patterns occur during high-density data recording on magnetic media, non-linear bit shifts arise. These bit shifts are primarily caused by demagnetization effects, i.e., as a result of the fact that the magnetic medium `opposes` the recording of a change of sign in the bit stream, particularly when a plurality of bits of the same sign precede one bit of an opposite sign. This effect manifests itself in that a change of sign or transition is recorded too late or too early, which results in bits being recorded which are too short or too long, respectively.
Another cause of non-linear bit shifts is the limited bandwidth of the recording channel. In the present description, a recording channel is to be understood to mean the signal path up to and including the write head, which also includes the write amplifier and the transformation of an electric input signal into a magnetic output signal in the write head. A data transition causes a comparatively slow change of the magnetic field in the write head by which the signals are recorded on the magnetic medium. The magnetization of the medium is not reversed and the transition is not recorded until the magnetic field strength exceeds a given threshold value. At high data rates, the total response of the magnetic field in the head to a signal transition extends over many bit intervals and thereby affects the recording of the transitions of many subsequent bits. In practice, it has been found that this effect can give rise to transitions being shifted with respect to one another by 20% or more of a bit interval.
Such inaccuracies in the positions of transitions relative to one another as a result of non-linear bit shifts are highly undesirable, particularly at high recording and read densities, because the reliability of the bit detection in the read signal is then reduced considerably.
2. Description of the Related Art
It is known that non-linear bit shifting as a result of demagnetization effects can be prevented, at least partly, by the use of write precompensation, which means that a sign change is either advanced or postponed depending on the values of a number of preceding bits. To this end, it is also known to consider either only the last preceding bit, a so-called first-order compensation, or the last two preceding bits, a so-called second-order compensation.
U.S. Pat. No. 4,607,295 describes a device comprising a transversal filter and by means of which, depending on the sign of two preceding bits, a choice is made for not delaying a bit to be recorded or for delaying this bit by a first or a second fixed delay.
U.S. Pat. No. 4,964,107 describes a device by means of which, depending on the sign of two preceding bits, it is decided to delay or not to delay the bit to be recorded. The selected delay then also depends on the position where the data is recorded on a magnetic disc, for which a choice can be made from eight delay values.
Finally, U.S. Pat. No. 5,325,241 describes a write precompensation device where the output voltage of an integrator formed by a capacitance is compared with three voltage values. Depending on which of the three voltage values is equal to the integrator voltage, a bit to be recorded is not delayed, is delayed by a first time interval or is delayed by a second time interval.
A disadvantage of these known write precompensation devices is that they only allow a choice from a very limited number of delay values for the compensation of non-linear bit shifts as a result of demagnetization effects, allowance being made for only a very limited number of preceding bits.
The article "Considerations for High-Data-Rate Recording With Thin-Film Heads" by R. Wood, M. Willians and J. Hong in IEEE Trans. Magn., Vol 26, No. 6, pp. 2954-2959, November 1990, describes that the limited bandwidth of the recording channel leads to bit shifts similar to the demagnetization effects. This is illustrated particularly by FIG. 9 of this article.
The article "Write current equalization for high speed digital magnetic recording" by T. Kato, R. Arai and S. Takanami in IEEE Trans. Magn., Vol. MAG-22, No. 5, pp. 1212-1214, September 1986, describes the problem of non-linear bit shifts as a result of the limited bandwidth of the recording channel, particularly as a result of the self-inductance of the write head. As a solution, it is proposed to use a filter which emphasizes the high frequencies. However, a disadvantage of this solution is that it can be used only at low bit rates. This can be explained as follows. The magnetic flux generated by the write head is proportional to the write current I. The write head comprises a coil having a self-inductance L, as a result of which the required write voltage is initially given by V=L.dI/dt. At an increasing bit rate, dI/dt will increase and, as a consequence, the required write voltage will also increase. In IC technology, this voltage is limited to 5 to 8 V, and this limitation dictates the maximum attainable bit rate during writing. In general, the use of a write equalizer of the type described in said article cannot palliate this limitation and, in practice, write equalization can therefore be used only at comparatively low bit rates.
The article "High data rate magnetic recording in a single channel" by C. H. Coleman et al. in the Journal of the Institution of Electronic and Radio Engineers, Vol. 55, No. 6, pp. 229-236, June 1995, also describes the problem of the slow response of the write head and proposes write equalization as a solution.
The European Patent Application 96202427.9, not published at the time of filing of the present Application, corresponding to U.S. patent application Ser. No. 08/909,919, filed Aug. 12, 1999, now U.S. Pat. No. 6,091,558, proposes a write compensation which enables an arbitrary number of time shifts to be selected for the compensation of the time shift caused both by demagnetization effects and by the limited bandwidth of the recording channel. This write precompensation circuit utilizes, for example, a transversal filter which, for any combination of a number of preceding symbol values, supplies a signal by means of which a given time delay can be obtained for a symbol to be recorded. In this prior proposal, allowance is made for four preceding symbols and one future symbol for the effect caused by the limited bandwidth.
However, the effect of the bandwidth limitation can be caused by a substantially larger number of preceding symbols which, if allowance is to be made for this in the prior-art device, would render this device quite extensive and, consequently, intricate.